The present invention relates to a power supply for generating a d.c. voltage from a sinusoidal a.c. voltage such as a commercial a.c. voltage.
A power supply for generating a d.c. voltage from the commercial a.c. voltage is, in general, arranged to have a rectifier circuit for rectifying the a.c. voltage and a capacitor (smoothing capacitor) for smoothing the voltage output from the rectifier circuit. The voltage charged in the smoothing capacitor matches to the output voltage of the power supply.
However, the a.c. load current of the commercial a.c. power supply flows only if the a.c. voltage exceeds the voltage charged in the smoothing capacitor, that is, it does not flow if the a.c. voltage is equal to or lower than the charged voltage. Hence, the power supply having such arrangement cannot generate a sinusoidal voltage following the a.c. voltage but a pulsewise wave voltage containing many harmonic components. It results in lowering a power factor of the commercial a.c. power supply, thereby having an adverse effect on the other instruments connected to the commercial a.c. power supply.
To overcome this shortcoming, several kinds of power supplies have been conventionally proposed for enhancing a power factor of the commercial a.c. power supply by controlling load current to be similar to a sine wave. Some conventional power supplies will be described.
FIG. 13 is a circuit diagram showing a full wave rectifier disclosed in JP-B-63-22148. In FIG. 13, 1 is an a.c. power supply, 2 is an inductor, 3 is a full wave rectifier circuit, 4 is a smoothing capacitor, 5 is load, D.sub.1 to D.sub.6 are diodes, T is a transistor, and R.sub.S1 to R.sub.d1 are resistors for detecting current.
As shown, a sinusoidal a.c. supply voltage V.sub.s supplied from the a.c. power supply 1 (see FIG. 14A) is supplied to the full wave rectifier circuit 3 composed of diodes D.sub.1 to D.sub.4 through the inductor 2. The supply voltage V.sub.s is rectified in the full wave rectifier circuit 3 and then is smoothed in the smoothing capacitor, finally being applied to the load 5.
If only the foregoing operation is done, the a.c. power supply 1 serves to flow a.c. load current I.sub.s only if the output voltage of the full wave rectifier circuit 3 is higher than the voltage charged in the smoothing capacitor 4. Hence, this a.c. load current I.sub.s has a pulsewise waveform synchronized with a positive and a negative peaks of the a.c. supply voltage V.sub.s as shown in FIG. 14(b), resulting in lowering a power factor of the a.c. power supply 1.
The arrangement shown in FIG. 13 provides the diodes D.sub.5 and D.sub.6 at the a.c. terminals of the full wave rectifier circuit 3. Those diodes D.sub.5 and D.sub.6 and the other diodes D.sub.3 and D.sub.4 compose an auxiliary full wave rectifier circuit and the output voltage of the auxiliary full wave rectifier circuit is chopped by a transistor T.
The transistor T is controlled on and off in response to a driving signal D.sub.rive (see FIG. 14(f)) composed of an on/off signal C.sub.h (see FIG. 14(d)) having a far higher frequency than the a.c. supply voltage V.sub.s and a period control signal V.sub.SP (see FIG. 14(e)) representing that the a.c. load current I.sub.s is in the range of -I.sub.SN &lt;I.sub.S &lt;I.sub.SP.
This arrangement results in allowing the a.c. load current I.sub.s to flow based on the on/off operation of the transistor T even during the period when the output voltage of the full wave rectifier circuit 3 is lower than the voltage charged in the smoothing capacitor 4. As shown in FIG. 14 (c), hence, the waveform of the a.c. load current Is is closer to the waveform shown in FIG. 14(b). It results in improving a power factor of the a.c. power supply 1.
The resistors R.sub.s1 and R.sub.d1 for sensing the current in order to detect the change of load. Depending on the sensed output of the resistors, a generator (not shown) for the driving signal D.sub.rive (see FIG. 14(f)) is controlled in order to adjust the period control signal V.sub.SP (see FIG. 14(c)), therefore, the reference values I.sub.SP and I.sub.SN (FIG. 14(c)).
FIG. 15 is a circuit diagram showing a power supply employing a voltage doubler rectifier circuit, which is disclosed in JP-B-63-22148. In FIG. 15, 4A and 4B are smoothing capacitors, 6 is a voltage doubler rectifier circuit, D.sub.7 to D.sub.10 are diodes, T.sub.A and T.sub.B are transistors, R.sub.s2 and R.sub.d2 are resistors for sensing current. The components corresponding to those shown in FIG. 13 have the same reference numbers.
In FIG. 15, the a.c. power voltage V.sub.s output from the a.c. power supply 1 has a polarity indicated by an arrow. The a.c. load current I.sub.s is conducted from the a.c. power supply 1 to the inductor 2, the diode 7, the smoothing capacitor 4A, and the resistor R.sub.S2, so that the smoothing capacitor 4A is charged with the voltage at the arrow-indicated polarity. If the a.c. supply voltage V.sub.s is at an opposite polarity to the arrow, the a.c. load current I.sub.s is conducted from the a.c. power supply 1 to the resistor R.sub.s2, the smoothing capacitor 4B, the diode D.sub.8, and the inductor 2, so that the smoothing capacitor 4B is charged with the voltage at the arrow-indicated polarity.
It results in applying the addition of the charged voltages of the smoothing capacitors 4A and 4B into the load 5 as a d.c. supply voltage.
Like the power supply shown in FIG. 13, this power supply does not conduct the a.c. load current I.sub.s during the period when the a.c. supply voltage V.sub.s is equal to or lower than the voltage charged in the smoothing capacitor 4A or 4B. Hence, the a.c. load current I.sub.s has a pulsewise waveform shown in FIG. 14(b).
To overcome this shortcoming, this power supply provides a circuit composed of both the diode D.sub.9 and the transistor T.sub.A connected in parallel to both the diode D.sub.7 and the smoothing capacitor 4A and another circuit composed of both the diode D.sub.10 and the transistor T.sub.B connected in parallel to both the diode D.sub.8 and the smoothing capacitor 4B. If the a.c. supply voltage V.sub.s is at a polarity indicated by an arrow, like the transistor T shown in FIG. 13, the a.c. supply voltage V.sub.s is chopped by driving the transistor T.sub.A on and off. If the a.c. supply voltage is at an opposite polarity to the arrow-indicated polarity, the a.c. supply voltage is chopped by operating the transistor T.sub.B on and off.
The chopped power voltage results in having a waveform closer to a sine wave as shown in FIG. 14(c), thereby enhancing a power factor of the a.c. power supply 1.
FIG. 16 is a circuit diagram showing a power supply employing a voltage doubler rectifier circuit, which is disclosed in JP-B-62-45794. In FIG. 16, 2A and 2B are inductors, 7 is a current sensor, 8 is a hysteresis-added comparator, 9 is a driving circuit, and D.sub.11 to D.sub.14 are diodes. The components corresponding to those shown in FIG. 15 have the same numbers.
If the a.c. supply voltage V.sub.s supplied from the a.c. power supply 1 is at a polarity indicated by an arrow, the a.c. load current I.sub.s is flown from the a.c. power supply 1 to the inductors 2A, the diodes D.sub.11 and D.sub.12, the smoothing capacitor 4A, and the inductor 2B, so that the smoothing capacitor 4A is charged with the voltage. If the a.c. supply voltage V.sub.s is at an opposite polarity to the arrow-indicated polarity, the a.c. load current V.sub.s is conducted from the a.c. power supply 1 to the inductor 2B, the smoothing capacitor 4B, the diodes D.sub.14 and D.sub.13, and the inductor 2A, so that the smoothing capacitor 4B is charged with the voltage. This results in applying additional voltage charges in the smoothing capacitors 4A and 4B as an a.c. supply voltage to the load 5.
An npn type transistor T.sub.A is provided in parallel to the diode D.sub.12 and the smoothing capacitor 4A and a pnp type transistor T.sub.B is provided in parallel to the diode D.sub.14 and the smoothing capacitor 4B. The hysteresis-added comparator 8 serves to compare the current flowing through the inductor with the current sensed by the current sensor 7. Based on the compared result, as shown in FIGS. 17(a) and 17(b), the driving circuit 9 is fixed at "L" (low level) if the a.c. supply voltage V.sub.s at the arrow-indicated polarity is equal to or higher than a predetermined level V.sub.1 and is fixed at "H" (high level) if the a.c. supply voltage V.sub.s at the opposite polarity to the arrow-indicated polarity is equal to or lower than a predetermined level V.sub.2. The driving circuit 9 serves to produce a driving signal reversing "H" to "L" or vice versa from the other period high frequency. Based on the driving signal, the transistors T.sub.A and T.sub.B are controlled on and off.
As will be understood from the above description, if the a.c. supply voltage Vs has the arrow-indicated polarity and is equal to or lower than the voltage charged in the smoothing capacitor 4A, the transistor T.sub.A is driven on and off for chopping the a.c. supply voltage V.sub.s. If the a.c. supply voltage V.sub.s is at the opposite polarity to the arrow-indicated polarity and is equal to or higher than the voltage charged in the smoothing capacitor 4B, the transistor T.sub.B is driven on and off for chopping the a.c. supply voltage V.sub.s.
Like the prior art shown in FIG. 15, therefore, this prior art can provide the a.c. load current I.sub.s having a waveform closer to a sine wave as shown in FIG. 17(a) from the sinusoidal a.c. supply voltage V.sub.s. It results in enhancing a power factor of the a.c. power supply 1.
The aforementioned prior arts, however, have the following problems.
(1) In the prior arts shown in FIGS. 15 and 16, if the a.c. load current I.sub.s is in the range from the reference value I.sub.sp to I.sub.sn, the waveform of the a.c. load current I.sub.s is not constantly sinusoidal, because the waveform depends merely on the conduction ratio of the transistors T, T.sub.A and T.sub.B.
In the prior art shown in FIG. 16, depending on the magnitude of the load, the reference values change in a predetermined sinusoidal manner as shown by a broken line and a two-dot chain line shown in FIG. 17. Hence, the a.c. load current I.sub.s can have a relatively excellent sinusoidal waveform, resulting in being able to reduce the harmonic wave and enhance a power factor. Since, however, the transistors T.sub.A and T.sub.B are controlled on and off depending on the above-mentioned reference values only, there exists a period when the switching is carried out at a high frequency without defining the chopping frequency. It results in causing an impractically large switching loss.
(2) In the aforementioned prior arts, as the switching frequency for the transistor T, T.sub.A or T.sub.B becomes higher, the waveform of the a.c. load current I.sub.s is made more sinusoidal. In this case, however, the diodes D.sub.1 and D.sub.2 shown in FIG. 13, the diodes D.sub.7 and D.sub.8 shown in FIG. 15, or the diodes D.sub.12 and D.sub.14 shown in FIG. 16 may be delayed due to the forward-biased or reverse-biased state. Those diodes serve as a capacitive load when the transistors T, T.sub.A and T.sub.B are switched on. On the other hand, when the transistors T, T.sub.A and T.sub.B are switched off, the inductor serves as an inductive load until the diodes enter into the forward-biased state, resulting in flowing excessive current, thereby increasing the switching loss. As a result, though the average a.c. load current is small, the transistors T, T.sub.A and T.sub.B are required to have large capacitance, because it is necessary to consider the switching loss.
(3) As will be apparent from the above description, to obtain a sinusoidal a.c. load current from the foregoing prior arts, as the on/off switching frequency for the transistor T, T.sub.A or T.sub.B becomes higher, the waveform of the a.c. load current becomes more precisely sinusoidal. Further, to reduce the inductor and the capacitor in size, it is necessary to perform the high-frequency switching operation. In the above-mentioned prior arts, however, it is necessary to perform the high-frequency switching operation of large current flown when the voltage is high for charging the smoothing capacitors, resulting in enlarging the switching loss. Hence, the conventional circuits have difficulty in performing the high-frequency switching operation.